*2.5. Vacuum Plasma Coating Composition Selection and Deposition on Solid Ceramic End Mills*

The choice of coating deposition method for SiAlON ceramic samples was made in favor of the well-proven technology of physical coating deposition of an evaporated material from a vacuum-arc discharge plasma. This technology makes it possible to form multicomponent coatings of the required composition with high productivity and reproducibility [50–55]. The coating was carried out on a STANKIN-APP technological unit prototype (MSTU Stankin, Moscow, Russia). Figure 6 shows a schematic diagram of a technological unit. The unit provides deposition of various compositions' coatings by varying the composition of the gas mixture supplied to the vacuum chamber through a multi-channel gas injection system and replacing cathodes, the outer surface of which has the shape of a right cone frustum. The formation of plasma by a vacuum arc discharge at a reduced pressure of various gases is accompanied by the appearance of a micro-droplet fraction and a neutral atomic and molecular component of the cathode material's erosion products [56–61]. The unit is equipped with a plasma flow filtration system to eliminate this drawback. A powerful electromagnetic field deflects ions, and microdroplets and neutral particles are captured and removed from the chamber, making it possible to form higher quality coatings on the ceramic samples' surfaces.

Three variations of coatings for deposition on SiAlON ceramic samples, which should contact with nickel alloys, were chosen based on the results of the authors' last works [62–70]: (CrAlSi)N—single-layer coating; (TiAl)N—sandwich-type multilayer coating; and (CrAlSi)N/ DLC—diamond-like carbon two-layer coating (this coating was developed by Platit AG, Selzach, Switzerland). The total thickness of the coatings was in the range of 3.5–3.9 μm. The ceramic samples were thoroughly cleaned in an ultrasonic tank using a soap solution at a temperature of 60 ◦C for 20 min and in alcohol for 5 min before coating deposition.

**Figure 6.** A technological unit's principal scheme for coatings' deposition on ceramic samples is using physical deposition of evaporated material from the vacuum arc discharge plasma.

Tribological tests were carried out on disk-shaped sintered specimens with three coating options to assess the prospects of the selected compositions' coatings to increase the wear resistance of SiAlON ceramics. Considering that the coated ceramic tool is expected to operate at elevated cutting temperatures, the tests were carried out both at room temperature and heated to 800 ◦C. The results of the high-temperature tests were of crucial importance for the selection of the coating, which was subsequently applied to the samples of solid ceramic end mills. The tests were carried out on a TNT tribometer by Anton Paar TriTec (Corcelles-Cormondrèche, Switzerland) using the "ball-on-disk" method with the rotation of the sample relative to a stationary counter body (nickel alloy ball), set at a distance relative to the rotation axis of the sample [71]. Table 3 shows the results of tribological tests with a friction length of 250 m, an applied load of 1 N, and a sliding speed of 10 cm/s.


**Table 3.** Results of preliminary tribological tests of 80% (90α10β) + 20% TiN ceramic specimens with different coatings.

The coating choice was made in favor of a (CrAlSi)N/DLC two-layer coating based on the experimental data obtained (hereinafter referred to as the DLC coating) since it demonstrated the lowest coefficient of friction and wear-track depth in contact with the counter body during high-temperature heating. It makes it possible to expect that this coating will reduce the intensity of the frictional and adhesive interactions of the cutting tool during milling nickel alloys.

The DLC coating deposition process on solid ceramic end mills (experimental and commercial end mills), implemented in the unit shown in Figure 6, was based on several innovative solutions of the current work's authors and well-known solutions of Platit AG (Selzach, Switzerland) and provided for the sequential implementation of the complex of the steps listed below:

	- (1) formation of a gradient layer: a gas mixture of 20% (Ar), 73% (N2), and 7% (Si(CH3)4), pressure in the chamber of 1.5 Pa, temperature in the chamber of 180 ◦C, rotation speed table of 5 rpm, negative bias voltage on the table of 500 V, and holding time of 20 min;
	- (2) formation of a diamond-like carbon layer: a gas mixture of 2% (Si(CH3)4), 55% (Ar), and 43% (C2H2), pressure in the chamber of 0.8 Pa, process temperature of 180 ◦C, table rotation speed of 5 rpm, negative bias voltage on the table of 500 V, and holding time of 100 min.
